The present invention relates to a hydrodynamic gas bearing structure and, more specifically, to a hydrodynamic gas bearing structure supporting a rotator rotating at a high speed.
Recently, as storage capacity has been increased and access time has been reduced in a rotation driving part of a magnetic recording apparatus, for example, a hard disc drive (hereinafter referred to as xe2x80x9cHDDxe2x80x9d), corresponding high rotational speed and high rotational accuracy have been required of a spindle motor for driving the HDD. In order to rotate a precision motor of which such a high rotational speed and high rotational accuracy are required at high speed, use of an air bearing (hydrodynamic gas bearing) in the rotation driving part has been proposed.
In the rotation driving part employing the air bearing, when the rotator rotates, air is forced into a space at least between a radial gas bearing body and the rotator. Accordingly, air pressure in the space increases, and the rotator rotates at high speed with air bearing interposed. In this manner, by the use of the air bearing, it is expected that rotational accuracy is maintained even during high speed rotation.
Generally, self-induced vibration associated with an air film resulting from dynamic pressure effect, referred to as xc2xd whirl, generates in the hydrodynamic gas bearing, which causes seizure of the bearing. In order to solve this problem, a technique has been proposed in which a herringbone shaped groove is formed in the shaft body or the bearing body to improve load capacity and stability, as described, for example, in Gas Bearing by Shinichi Tohgo, Kyoritsu Shuppan (1984). Further, a technique has been proposed in which a space varying portion is provided in the bearing as disclosed in Japanese Patent Laying-Open No. 8-312639.
The hydrodynamic gas bearing adjusted in the above described manner, however, has the following problems.
For example, Japanese Patent Laying-Open No. 8-312639 describes that a bearing free from generation of xc2xd whirl and having superior rotational characteristic can be obtained by providing a space varying portion in the space formed between the shaft body and the bearing body and by setting out-of-roundness (concave and convex deviation with respect to mean diameter) of the shaft body and the bearing body with respect to respective diameters to at most 3xc3x9710xe2x88x924.
In the actual manufacturing, especially in internal processing of a hollow cylindrical body, however, when finishing process is performed by honing or inner periphery grinding using a grinder, and a motor incorporating the thus processed bearing is rotated, run out occurs, presenting practical problem.
An object of the present invention is to provide a hydrodynamic gas bearing structure suitable for actual use, capable of maintaining high rotational accuracy at a high speed rotation and suppressing run out associated with rotation.
As a result of intensive study of the hydrodynamic gas bearing structure, the inventors have found that when cross sectional shape of sliding surface of the shaft body and the bearing body has a prescribed convex polygonal shape, xc2xd whirl does not generate even at the speed of rotation of 20,000 rpm or higher and run out can be suppressed.
Therefore, the hydrodynamic gas bearing structure in accordance with the present invention includes a columnar shaft body and a hollow cylindrical bearing body opposing to the shaft body with a radial space kept therebetween, and the structure has the following features. When the shaft body and the bearing body are arranged so that central axes thereof are aligned, an approximately cylindrical space is formed by the outer peripheral surface of the shaft body and the inner peripheral surface of the bearing body. In a cross sectional shape perpendicular or orthogonal to the axis of the shaft body and the bearing body, either the shaft body or the bearing body has a convex polygonal shape, with the convex polygonal shape having ten or more vertexes.
Thus, run out can be suppressed as compared with the prior art, and therefore the present invention can be applied without any practical problem to applications requiring high speed of rotation and high rotational accuracy.
Further, symmetry of the space with respect to the radius can be improved, attaining symmetrical pressure distribution in the space. Accordingly, a lubrication film derived from dynamic pressure effect is formed sufficiently, whereby direct contact between the shaft body and the bearing body is prevented, and hence seizure can be prevented.
When the hydrodynamic gas bearing structure has a structure as described above and the ratio of the out-of-roundness of the shaft body and the bearing body with respect to the diameter is at most 1xc3x9710xe2x88x924, symmetry of the space with respect to the radial direction can further be improved when the shaft body and the bearing body are arranged concentrically, and the fluid flow in the space is averaged. Accordingly, symmetry of pressure distribution in the space can further be improved, suppressing local variation in pressure. Therefore, insufficient lubrication caused by uneven pressure in the space can be suppressed and hence run out is reduced.
The inventors have further found that when the hydrodynamic gas bearing structure has the above described structure and a space enlarging portion for enlarging the space is provided on at least one of the shaft body and the bearing body, it becomes possible to suppress xc2xd whirl and to improve rotational accuracy even at a high speed rotation with the speed of 20,000 rpm or higher.
Therefore, in a preferred aspect of the present invention, at least one space enlarging portion is provided extending in the axial direction, for enlarging the space, in at least one of the shaft body and the bearing body.
The amount of enlargement of the space at the space enlarging portion is preferably at least 0.2 and at most 1 times the thickness of the space other than the space enlarging portion (hereinafter referred to as average gap). When the amount of enlargement of the space at the space enlarging portion is defined in this manner, suppression of xc2xd whirl and improvement of rotational accuracy at high speed rotation can more effectively be attained.
The inventors have found that in another preferred aspect of the present invention, at least one axial end of the space enlarging portion is formed from a region where the shaft body and the bearing body oppose to each other to a region where the shaft body and the bearing body do not oppose to each other. Thus, supply of fluid from outside the bearing to the space of the bearing is facilitated, the supporting force of the fluid film is secured by the effect of the space enlarging portion, and local variation of pressure can be suppressed by the effect of the convex polygonal space, whereby xc2xd whirl generated at a high speed rotation of 20,000 rpm or higher can be suppressed and run out in synchronization with the rotation can be reduced.
According to another preferred aspect of the present invention, one of the shaft body and the bearing body has a plurality of space enlarging portions, and the plurality of space enlarging portions are arranged at equal distance from each other in cross section. Thus, the above described effect of symmetry in the peripheral direction realized by the provision of the space enlarging portions is further enhanced.
According to another preferred aspect of the present invention, the space enlarging portion is stationary or rotational with respect to an external stationary system. Thus, the space enlarging portion provides the above described effect no matter whether it is placed on the rotational side or stationary side.
In accordance with a still further preferred aspect of the present invention, with respect to the gas flow in the peripheral direction of the space, space variation ratio on the gas inlet side of the space enlarging portion is larger than the space variation ratio on the gas outlet side. Accordingly, the efficiency of dynamic pressure generation is improved, and hence run out can be reduced.
In another preferred aspect of the present invention, the ratio of the space variation ratio on the inlet side with respect to the space variation ratio on the outlet side is smaller than 10. Thus, run out can further be suppressed.
According to another preferred aspect of the present invention, the space enlarging portion has a large space portion where the amount of enlargement of the space is relatively large, and a small space portion where the amount of enlargement of the space is relatively small. The large space portion is positioned on the gas inlet side of the space enlarging portion, and the small space portion is arranged on the gas outlet side, with respect to the gas flow in the peripheral direction of the space. Thus, the efficiency in generating dynamic pressure in the space enlarging portion is enhanced, and therefore run out can further be suppressed.
In another preferred aspect of the present invention, the ratio of the amount of enlargement of the space of the large space portion with respect to the amount of enlargement of the space in the small space portion is at most 20. Thus, run out can further be suppressed.
In another preferred aspect of the present invention, the space variation ratio on the inlet side of the large space portion is larger than the space variation ratio at the outlet side of the small space portion. Thus, efficiency in generating dynamic pressure can be improved, and hence run out can be reduced.
In another preferred aspect of the present invention, the ratio of the space variation ratio at the inlet side of the large space portion with respect to the space variation ratio at the outlet side of the small space portion is smaller than 10. Thus run out can further be suppressed.
In another preferred aspect of the present invention, the ratio of the amount of enlargement of the space at the large space portion with respect to the amount of enlargement of the space at the small space portion is at most 20. Thus, run out can further be suppressed.
In another preferred aspect of the present invention, the length in the axial direction of the space enlarging portion is at least ⅓ of the length in the axial direction of the region where the outer peripheral surface of the shaft body opposes to the inner peripheral surface of the bearing body. Thus, it becomes possible to prevent abnormal vibration at a high rotation area, and to suppress direct sliding at the time of starting and stopping rotation to a lower number of rotation. Accordingly, a bearing structure and a motor having long life reliability can be obtained.
More preferably, the length in the axial direction of the space enlarging portion is at most ⅔ of the length in the axial direction of the region where the outer peripheral surface of the shaft body opposes to the inner peripheral surface of the bearing body. Thus, the effect described above can further be enhanced.